This invention related to a novel method for the production of presensitized lithographic printing plates and to the novel lithographic printing plates produced thereby. More particularly, this invention related to a novel method of producing aluminum based photosensitive lithographic printing plates which comprises coating at least one surface of an aluminum base sheet with a lithographically suitable, light-sensitive diazo composition, and finally applying to said surface a lithographically suitable photopolymerizable coating to yield the desired photosensitive lithographic printing plate.
It is well recognized that the substantial majority of photosensitive lithographic printing plates now produced are made from an aluminum based sheeting. This aluminum sheeting has, over the years, been found to suffer from certain disadvantages which prevent the direct application of the light-sensitive coating to the aluminum base sheet. It has been found that upon direct application of the light-sensitive composition to the aluminum base sheet, and subsequent exposure thereof to light, and the removal of the non-imaged areas for the preparation of a lithographic printing plate, the resultant printing plate has many undesirable characteristics which render it unacceptable for commercial use in the printing industry. Among the disadvantages suffered by such plates can be included the fact that the removed non-image areas do not possess sufficient hydrophilic and oleophobi characteristics, and therefore on printing will pick up background imperfections which will be imprinted on the final copy. In addition, since the aluminum is a rather soft metal, it does not have the resistance properties to sustain long press runs where more than about 100,000 copies are required. Further, the characteristic of the aluminum surface is such that a problem is encountered in achieving a strong bond between the light sensitive composition and the aluminum base sheet thus causing the image area of the plate to be dislodged from the surface, and renders the printed copy imperfect.
Heretofore, in the production of metal presensitized lithographic printing plates, it had been found beneficial to treat the surface of the metal substrate sheet, with a protective interlayer substance which imparts beneficial characteristics to the final lithographic printing plate thus produced. The prior art teaches that it is desirable to treat the metal sheet substrate surface receiving the light sensitive coating material, which when exposed to light and developed becomes the printing surface of the printing plate, with an undercoating substance that forms a strong bond with the metal sheet substrate and with the light sensitive coating material.
Many such undercoating treatments are known in the art for manufacturing longer running lithographic plates, and can be used on the sheets of this invention. U.S. Pat. Nos. 3,160,506, 3,136,636, 2,946,683, 2,922,715, disclose a variety of suitable materials for undercoating bonding substances onto plates and methods for applying them. Alkali metal silicate, silici, acid, alkali zirconium fluoride and hydrofluozirconic acid solutions presently are the most important commercial bonding substances. Those materials substantially improve the bonding of the light sensitive coating to the underlying metallic base which otherwise generally tends to have inadequate affinity for the coating. Of the various known bonding mterials, the Group IV-B metal fluorides, the alkali metal salts and the acids thereof are preferred. In particular, the alkali zirconium fluorides, such as potassium zirconium hexafluoride, and hydrofluozirconic acid disclosed in U.S. Pat. Nos. 3,160,506 and 2,946,683 are used for preparing aluminum bases to receive a light-sensitive coating.
One method in particular which is disclosed in the Jewett et al. U.S. Pat. No. 2,714,066 issued July 26, 1955, involves the use of a silicious overcoating on the aluminum sheet surface which acts as an interlayer bonding the photosensitive composition to the aluminum sheet surface. However, it has been found that this method does not completely solve the problem of bonding strength in that the bonding between the photosensitive composition and the aluminum base sheet has been found not to be strong enough to withstand long press runs and, in addition, tends to wear out or fail over an extended period of time.
The instant invention provides a novel method of manufacturing presensitized printing plates wherein a photosensitive, lithographically suitable, preferably water-soluble diazo type composition is applied directly to the surface of a metal sheet substrate, preferably aluminum although other metals such as zinc or copper may also be employed. Said metal sheet substrate may have been previously treated in a variety of ways known in the art, such as graining, etching or anodizing treatments or may have been provided with an interlayer composition as hereinbefore mentioned. The diazo coated substrate is then subsequently coated with a photosensitive, solvent soluble, lithographically suitable photopolymerizable composition.
The resulting lithographic plate demonstrates a substantially extended shelf life, a protracted press life and improved clarity of the resultant printed image.